1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst, in particular, to an exhaust gas-purifying catalyst including hydrocarbon-adsorbing material.
2. Description of the Related Art
Until today, as an exhaust-gas purifying catalyst for treating an exhaust gas from an automobile, etc., used widely is a three-way catalyst in which a refractory carrier made of an inorganic oxide such as alumina supports a precious metal. In the three-way catalyst, the precious metal plays the role in promoting the reduction of nitrogen oxides (NOx) and the oxidations of carbon monoxide (CO) and hydrocarbons (HC). Further, the refractory carrier plays the roles in increasing the specific surface area of the precious metal and suppressing the sintering of the precious metal by dissipating heat generated by the reactions.
In some cases, a three-way catalyst is used in combination with a hydrocarbon-adsorbing material such as zeolite as described in JP-A 11-210451. In such a case where the hydrocarbon-adsorbing material is used, it is possible to allow HC to adsorb on the hydrocarbon-adsorbing material when the temperature of the catalyst is low, for example, when starting an engine; and to allow HC to desorb from the hydrocarbon-adsorbing material when the temperature of the catalyst is increased, so as to oxidize the desorbed HC by the catalysis of the precious metal. Therefore, using a hydrocarbon-adsorbing material is expected to decrease HC emission.
However, the present inventors have found in the course of achieving the present invention that an automotive vehicle having such a three-way catalyst is susceptible to improvement in terms of HC emission.